Steering angle sensors measure the turning angle of the steering wheel of a motor vehicle. Accordingly, under the term “steering angle” in the present description, the angle of rotation of the steering shaft and of the steering wheel coupled thereto is to be understood, not the turning angle of the steered wheels of the vehicle, however. Among other things, this information is required for the vehicle dynamics control, the electronic stability program (ESP). The general problem with such determinations of the angle of rotation is that the steering shaft makes a plurality of revolutions possible in order to move the steering system from steering stop to steering stop. Therefore, in the case of the most commonly used sensors, a continuous recording and saving of the current rotation angle position of the steering wheel is required, since common angle sensors can measure a maximum of 360°, however, a motor vehicle steering wheel has an angle range of +/−720° (four revolutions in total) or more.
In the publication DE 10 2008 011 448 A1, a solution is suggested where the steering shaft is detected by two gear transmissions, whereby a first gear transmission is a hypocycloid transmission, with which an absolute angle value can be determined via a plurality of revolutions. A second gear transmission is used to improve the angular resolution so that the angle of rotation can be more accurately determined. By means of the calculated combination of both detected angle signals, a determination of the absolute angle is made possible. The proposed solution, however, is particularly complex due to the use of two gearings and the calibration of the absolute angle is complex since the neutral position or straight position must first be found respectively.
DE 600 11 684 T2 discloses a solution where two sensors are used, of which one monitors the position of the steering shaft and the other monitors the position of the rotor of the servomotor in order to generate a clear measurement of the steering shaft angle across a range of angles beyond a full rotation. Thereby, an increasing or decreasing incrementation of revolutions takes place in order to determine an absolute angle by means of the combination of both signals. A disadvantage of this solution is the complex calibration of the measurement signal to the absolute angle.
An integral construction unit made of a steering angle sensor and a torque sensor is known from WO 2015/078664 A1, wherein the steering angle sensor has a gearing where the position of two gears is determined in order to calculate the absolute angle of steering.
Conventional systems also require a device that detects the steering angle even when ignition is switched off so that the correct steering angle is available upon starting the vehicle and the control system of the electromechanical power steering can function. Furthermore, using a conventional system, the measuring accuracy can only be achieved with great effort.
Thus a need exists for an electromechanical power steering system with a system for determining a highly-accurate absolute angle of its steering shaft, which can be easily calibrated with a high level of accuracy and which additionally has a simple and inexpensive construction.